MCB not working after a trip

[Richard D]

Looking for some advice before I call an electrician...

I accidentally tripped the circuit breaker when replacing a light fitting in my garage. The MCB for the garage tripped as well as the overall RCCB. (The trip was totally my fault and it proved the circuit protection devices did their job).

When I reset the RCCB the rest of the house is fine. When I switched the MCB for garage circuit back on it stayed on, but there is no power in the garage at all. I tried to reset the MCB again. It stays on no problem, but there is no power in the garage.

Has this trip damaged the MCB? Is there anything else I can check before calling someone out?

 

[crystal ball]

I would check all the connections in the garage circuit, the fault might have affected one or more

 

[JohnW2]

When I switched the MCB for garage circuit back on it stayed on, but there is no power in the garage at all.

With some MCBs (indeed, also some RCDs/RCCBs) one has to push the lever firmly right down before it will properly reset when then pushed up

Kind Regards, John

 

[Richard D]

With some MCBs (indeed, also some RCDs/RCCBs) one has to push the lever firmly right down before it will properly reset when then pushed up

Kind Regards, John

Thanks John. I have read that elsewhere, although on my consumer unit, I cannot see any way of pushing the switch any further down than I am already doing.

 

[Richard D]

I would check all the connections in the garage circuit, the fault might have affected one or more

Thanks. The circuits within the garage are actually very simple. The wire comes from the house into the back of a double socket. From there, there is a fused spur to the lighting circuit and another spur to a further double socket. Looking in the back of the first socket, there is no evidence of any problem.

 

[JohnD]

As crystal ball says.

quite likely a loose connection somewhere burned out during the momentary surge.

Use a multimeter and track all the connections in sockets, switches, ceiling roses etc, and in the CU. For best confidence, when you have found the faulty one, take it apart and remake it. If there is any heat-marked copper, cut it off back to bright metal.

Do the CU last because you will have to turn off the main switch, and there are live points inside even after that.

 

[JohnW2]

Thanks John. I have read that elsewhere, although on my consumer unit, I cannot see any way of pushing the switch any further down than I am already doing.

Fair enough, but I thought it was probably worth a try (if you hadn't already). The lever obviously won't 'go down further than it will go down' - the problem is that, with some of them, when they trip they don't actually go 'right down' (even though they may look as if they have), in which case a little downward push can enable them to be subsequently reset.

Do I take it that you're sure that there is not another MCB or fuse lurking somewhere which has also tripped/blown (for example, a fuse in an FCU in the garage)?

Kind Regards, John

 

[Richard D]

I am a bit confused (easily done). If there was a problem with any of the wiring in the garage - as a result of the surge - I'm surprised that the MCB is happy to reset?

 

[Richard D]

Fair enough, but I thought it was probably worth a try (if you hadn't already). The lever obviously won't 'go down further than it will go down' - the problem is that, with some of them, when they trip they don't actually go 'right down' (even though they may look as if they have), in which case a little downward push can enable them to be subsequently reset.

Do I take it that you're sure that there is not another MCB or fuse lurking somewhere which has also tripped/blown (for example, a fuse in an FCU in the garage)?

Kind Regards, John

A very good question! I also assumed there must be another failsafe device in the circuit, but as far as I can see, there is nothing between the consumer unit and the first socket in the garage. At least if there is, it's not at all obvious!

I think I will need to call someone to check the MCB in the consumer unit - to check there is current coming across it...

 

[JohnW2]

I am a bit confused (easily done). If there was a problem with any of the wiring in the garage - as a result of the surge - I'm surprised that the MCB is happy to reset?

Well, again, it might be 'going up' (and staying up) without actually 'resetting' (electrically).

However, that's the reason I just asked about the possibility of another MCB or fuse somewhere. What exactly have you got in the garage (electrically speaking), and do you know how it is 'arranged' (wired)?

Kind Regards, John

 

[Richard D]

^^^ The cable enters the garage and goes immediately into the back of a twin 13a socket. From this socket, there is a fused spur to a lighting circuit (a simple two-way with two strip lights) and another spur to an additional double socket.

 

[JohnW2]

^^^ The cable enters the garage and goes immediately into the back of a twin 13a socket. From this socket, there is a fused spur to a lighting circuit (a simple two-way with two strip lights) and another spur to an additional double socket.

Right. Well, given how the problem arose, it's quite possible that the fuse in the 'fused spur' (Fused Connection Unit, FCU) has blown, which would explain the lack of lighting - but that obviously would not explain a lack of power at the socket. Are you sure that the socket is 'dead'?

Kind Regards, John

 

[Richard D]

^^^ Thanks. Indeed, I have checked the socket - definitely no power. I also checked the fuse in the FCU, which was ok.

I'm fairly sure there is no power reaching the garage, so I guess the problem must be in the consumer unit.

 

[JohnW2]

^^^ Thanks. Indeed, I have checked the socket - definitely no power. I also checked the fuse in the FCU, which was ok.

Fair enough.

I'm fairly sure there is no power reaching the garage, so I guess the problem must be in the consumer unit.

It's certainly unlikely that any wiring will have been damaged as a result of what happened (particularly since the MCB tripped and therefore terminated the problem very quickly), so what you say seems the case - the most likely thing being that the experience has 'killed' the MCB. I suppose it's just about conceivable that a very poor connection somewhere could have failed completely as a result of the very brief, but probably very high, fault current, but I would think that was pretty unlikely.

Kind Regards, John

 

[Richard D]

Fair enough.
It's certainly unlikely that any wiring will have been damaged as a result of what happened (particularly since the MCB tripped and therefore terminated the problem very quickly), so what you say seems the case - the most likely thing being that the experience has 'killed' the MCB. I suppose it's just about conceivable that a very poor connection somewhere could have failed completely as a result of the very brief, but probably very high, fault current, but I would think that was pretty unlikely.

Kind Regards, John

Thanks, I think I will admit defeat and call someone to diagnose and fix the problem. Can't tell you how annoyed I am with myself for stupidly causing this situation, but on the bright side, I'm grateful the MCB did its job.